1. Field of the Invention
The present invention relates to optical metrology, and more particularly to resolution enhanced optical metrology.
2. Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine various characteristics, such as the profile of the structure. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a periodic grating in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the periodic grating. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated.
However, the resolution of conventional optical metrology may be limited. More particularly, consider a structure with a pitch p that is illuminated obliquely under a certain incidence angle θi. With the wavelength λ of the illumination, the diffraction order m propagates in a direction θm that can be computed by the grating equation in reflection:
                                          -            sin                    ⁢                                          ⁢                      θ            m                          =                              sin            ⁢                                                  ⁢                          θ              i                                +                      m            ·                          λ              p                                                          (        1        )            
The signs characterize the oppositeness of the angles, e.g., the zero reflection order propagates in −θi direction. Now, assume that the incident angle is chosen such that the diffraction in the minus first order is symmetric to the specular reflected beam, i.e., θ0=−θi=−θ−1. Then, the following is obtained from equation (1):
                              sin          ⁢                                          ⁢                      θ            i                          =                  λ                      2            ⁢            p                                              (        2        )            
Assuming that the structure is imaged by a lens with an aperture angle u and the lens is diffraction limited, the image resolution increases with increasing numerical aperture or aperture angle u of the lens with the theoretical limit u=90°. Inserting this value in equation (2), the Abbe resolution limit follows as:
                    p        =                  λ          2                                    (        3        )            
Theoretical and experimental investigations have shown that depending on the geometry and material of the structure, 3σ measurement precision less than 1% can be obtained for conventional optical metrology for structures with a minimum pitch p of:
                    p        =                  λ          M                                    (        4        )            where M is a real number, such as 3. However, feature geometries may shrink below this resolution limit.